Resource use efficiency of
Greenhouse production systems
Article & Photos Estelle Kempen
DEPARTMENT OF AGRONOMY, FACULTY OF AGRISCIENCES
STELLENBOSCH UNIVERSITY
T
he huge increase in the
global food demand anticipated for the next 50 years
necessitates that agriculturists manage the resources required
for food production in such a way
that the services ecosystems provide
to society remain sustainable. For
this reason a lot of research is being
done on the resource use efficiency
of production systems. The two
most popular descriptors are the
water use efficiency (WUE) and nutrient use efficiency (NUE) although
light use efficiency (LUE) is often
also examined. These terms simply
refer to the amount of water, nutrients or light that will be needed per
harvestable product produced.
Growing crops hydroponically under protection is a complex production system associated with a high
degree of control on inputs and
outputs. One of the big advantages
of this is that maximum yield will be
obtained from the crops, normally
also in a shorter production period
reducing the resources needed. By
keeping the temperature, relative
humidity and light intensity at the
optimum levels for your specific
crop you ensure that photosynthesis and therefore crop growth will
be optimal, and very little energy
needs to be used by the plant to
protect it against abiotic stress factors. When light intensity and temperatures increase significantly on a
hot summer’s day most plants will
close their stomata to protect themselves from losing too much water.
Unfortunately this means that no
CO2 will enter the leaf, subsequently
reducing
photosynthesis
and
growth. This can be prevented in a
climate controlled greenhouse.
Apart from controlling the environment, a great degree of control
over the rootzone is also possible. A
lot of research has been done on
the water and fertilizer needs of
many food crops and when growing crops in a grow bag with a substrate such as sawdust, coir or perlite we can very accurately control
the amount of water and nutrients
we apply to the plants. A large percentage of nutrients applied is however still lost and leached to the
environment. It is estimated that on
average 10-30 kg of N ha-1 per year
is lost in intensive open field production and in non-recirculating
soilless production systems up to
1000 kg N ha-1 per year is lost. Although the fertilizer and water cost
only makes up a small percentage
of the total cost component of
greenhouse farming, the environmental impact of these losses can
be substantial. The resource use
efficiency of the system can however be increased if we convert to a
closed system where the drained
fertigation water is re-used for fertigation.
In Europe it is obligatory to re-use
drained nutrient rich water from
greenhouses. However, in South
Africa most greenhouses only employ basic climatic control strategies
and drained nutrient rich water is
still mostly run to waste. The main
reasons that producers do not recycle their drained nutrient solutions
include fear of disease transmission
and nutrient imbalances that develop leading to reduced yields. When
re-using drained nutrient solution
the ion composition, measured as
the Electrical Conductivity (EC) will
start deviating from the optimal
starting solution as the time period
of recirculation increases. It is often
found that the Ca2+ and SO42- levels
especially will escalate while that of
NO4- and K+ will decrease. The nutrient solution therefore needs to be
monitored frequently and adjusted
appropriately. The department of
Agronomy at Stellenbosch University is currently researching methods
to predict these changes in the
leached nutrient solution to assist
growers in maintaining optimum
nutritional levels with the re-use of
their fertigation water. Results have
shown that even with a simple conversion of a normal drain to waste
system to a system where all the
drained nutrient solution is collected, sterilised, and re-used can result
on average in 30% reduction in water and fertiliser use. This will of
Litres of water needed to produce 1kg of the following foods
822
287
2497
5553
1608
17196